Since their discovery controlled radical polymerization (“CRP”) processes have gained increasing research and industrial attention. CRP processes couple the capability of conventional free radical polymerization (“RP”) to (co)polymerize a wide range of monomers with the ability to synthesize polymeric materials with predetermined molecular weight (“MW”), low polydispersity (“PDI”), controlled composition, site specific functionality, selected chain topology, and incorporation of biological or inorganic species into the final product.
The three most studied methods of CRP processes are nitroxide mediated polymerization (“NMP”), atom transfer radical polymerization (“ATRP”), and degenerative transfer with dithioesters via reversible addition-fragmentation chain transfer polymerization (“RAFT”). CRP processes typically, but not necessarily, comprise a relatively low stationary concentration of propagating chain ends in relation to dormant chain ends. A dormant chain end comprises a transferable atom or group. The dormant chain end may be converted to a propagating chain end by loss of the transferable atom or group to the transition metal complex in the lower oxidation state. The low concentration of propagating chain ends present during the polymerization process reduces the probability of bimolecular termination reactions, leading to radical polymerization processes that behave as a “living” polymerization process.
The ATRP equilibrium (characterized by KATRP) most frequently involves homolytic cleavage of an alkyl (pseudo)halide bond R—X by a transition metal complex activator Mtn/L which (reversibly) generates an active propagating alkyl radical R. and the corresponding higher oxidation state metal halide deactivator Mtn+1X/L in a redox reaction Scheme 1.

The active R• may then propagate with a vinyl monomer (M), be deactivated in this equilibrium reaction by Mtn+1X/L, or terminate by either coupling or disproportionation with another R•. Such termination results in an increase in the amount of deactivator, Mtn+1X/L, by two equivalents resulting in an increase in concentration of dormant species as a result of the persistent radical effect. [Fischer, H. Chem. Rev. 2001, 101, 3581-3610.]
In some embodiments of CRP processes, a fast rate of initiation (“Ri”), relative to the rate of propagation (“Rp”), (For example, where from a process where Ri<<Rp to a process where Ri˜Rp) contributes to control of the molecular weight, degree of polymerization (“DPn”) and molecular weight distribution. As used herein, DPn˜[M]/[I]0, where [M] is the moles of monomer polymerized and [I]0 is the initial concentration of the added initiator. Termination reactions will tend to reduce the control over such properties and since CRP processes are radical based polymerization processes, some termination reactions during a CRP process are unavoidable.
In all radical polymerizations, biradical termination occurs with a rate of termination (“kt”) which is dependent on the concentration of radicals (“[P*]”) to the power two (Rt=kt[P*]2). Therefore, it may be assumed that at the same rate of propagation (the same concentration of radicals), generally the same number of chains would terminate, regardless whether the polymerization process is a RP or a CRP. However, this assumption ignores the diffusion effect of the macromolecule radicals in a CRP. In a RP most chains are terminated by the reaction of a small radical with a growing polymer radical. In the case of SFRP, or ATRP, these initial termination reactions result in an increase in the concentration of dormant species as a result of the persistent radical effect, [Fischer, H. Chem. Rev. 2001, 101, 3581-3610.]
In an RP, all polymer chains are eventually terminated, whereas in CRP the terminated chains constitute only small fraction of all chains (˜1 to 10%) while most polymer chains are in the dormant state. The majority of polymer chains in a CRP in the dormant state are capable of reactivation which allows continuation of the polymerization, functionalization, chain extension to form block copolymers, etc. Thus, a CRP behaves as a “living” polymerization process. [Greszta, D. et. al. Macromolecules 1994, 27, 638.] As used herein, “polymer” refers to a macromolecule formed by the chemical union of monomers, typically five or more monomers. The term polymer includes homopolymers and copolymers including random copolymers, statistical copolymers, alternating copolymers, gradient copolymers, periodic copolymers, telechelic polymers and polymers of any topology or architecture including block copolymers, graft polymers, star polymers, bottle-brush polymers, comb polymers, branched or hyperbranched polymers, and such polymers tethered to particle surfaces or flat surfaces as well as other polymer structures.
ATRP is the most frequently used CRP technique with a significant commercial potential for many specialty materials including coatings, sealants, adhesives, dispersants but also materials for health and beauty products, electronics and biomedical applications. The most frequently used ATRP process comprises a reversible halogen atom transfer catalyzed by redox active transition metal compounds, most frequently copper based. ATRP transition metal catalysts typically comprise a transition metal complexed with a ligand. In ATRP, radically polymerizable monomers are polymerized in the presence of a transition metal catalyst. For a list of radically polymerizable monomers, see U.S. Pat. No. 5,763,548, hereby incorporated by reference. It is believed that the transition metal catalyst participates in a redox reaction with at least one of an ATRP initiator and a dormant polymer chain, see Scheme 1. Suitable transition metal catalysts comprise a transition metal and a ligand coordinated to the transition metal. The transition metal catalyst participates in a reversible redox reaction with at least one of an ATRP initiator and a dormant polymer chain. Suitable transition metal catalysts comprise a transition metal and, optionally, at least one ligand coordinated to the transition metal. The activity of the transition metal catalyst depends on the composition of the transition metal and the ligand.
To function as an ATRP transition metal catalyst, the transition metal must have at least two readily accessible oxidation states separated by one electron, a higher oxidation state and a lower oxidation state. The reversible redox reaction results in the transition metal catalyst cycling between the higher oxidation state (the “deactivator state”) and a lower oxidation state (the “activator state”) while the polymer chains cycle between having propagating chain ends and dormant chain ends. Typically, the transition metal is one of copper, iron, rhodium, nickel, cobalt, palladium, molybdenum, manganese, rhenium, or ruthenium. In some embodiments, the transition metal catalyst comprises a copper halide, and preferably the copper halide is one of Cu(I)Br or Cu(I)Cl. Living/controlled polymerizations typically, but not necessarily, comprise a relatively low stationary concentration of polymers comprising propagating chain ends in relation to polymers having dormant chain ends. When the polymer has a dormant chain end, the chain end comprises the transferable atom or group. The dormant chain end may be converted to a propagating chain end by transfer of the transferable atom or group to the transition metal catalyst. The description of the mechanism of an ATRP is provided for explanation and is not intended to limit the invention. The disclosed mechanism is generally accepted, but different transition metal catalyst may result in different mechanisms. The ligand affects the structure of the catalyst, the solubilizing effect, and catalyst activity. See Catalyst Development www.chem.cmuedu/groups/maty/about/research/05.html, hereby incorporated by reference.
ATRP is considered to be one of the most successful CRP and has been thoroughly described in a series of co-assigned U.S. patents and applications, such as U.S. Pat. Nos. 5,763,548; 5,807,937; 5,789,487; 5,945,491; 6,111,022; 6,121,371; 6,124,411; 6,162,882; 6,407,187; 6,512,060; 6,538,091; 6,541,580; 6,624,262; 6,624,263; 6,627,314; 6,759,491; and U.S. patent application Ser. Nos. 09/534,827; 09/972,056; 10/034,908; 10/269,556; 10/289,545; 10/638,584; 10/860,807; 10/684,137; 10/781,061 and 10/992,249 all of which are herein incorporated by reference. ATRP has also been discussed in numerous publications with Matyjaszewski as co-author and reviewed in several book chapters. [ACS Symp. Ser., 1998, 685; ACS Symp. Ser., 2000; 768; Chem. Rev. 2001, 101, 2921-2990; ACS Symp. Ser., 2003; 854; ACS Symp. Ser., 2006; 944.] Within these publications similar polymerization processes may be referred to by different names, such as transition metal mediated polymerization or atom transfer polymerization, but the processes may be similar and if involve reaction mechanism of Scheme 1 will be referred to herein as “ATRP”. Such publications describe ATRP catalysts including the reducing power of several transition metal ligand combinations and the manner in which an ATRP equilibrium can be adjusted for more or less reactive monomers.
Embodiments of ATRP processes provide advantages over other CRP processes, including the availability wide variety of initiators and macroinitiators, including wafers, inorganic colloids, glass, paper, and bio-active molecules including proteins, DNA, carbohydrates and many commercial polymers may be simply synthesized as initiators; many polymers produced by ATRP allow facile functionalization or transformation of the end groups by replacing terminal halogens with azides, amines, phosphines and other functionalities via nucleophilic substitution, radical addition or other radical combination reactions; an abundance of monomers are polymerizable by ATRP. Such monomers include, but are not limited to, styrenics, (meth)acrylates, acrylonitrile, acrylamides, vinyl chlorides, and other monomers. Embodiments of ATRP allow the production of macromolecules with complex topology such as stars, combs and dendrimers, coupled with the ability to control composition and hence functionality in block, gradient, periodic copolymers etc. and even control polymer tacticity. ATRP may be carried out in bulk, or in the presence of organic solvents or in water under homogeneous or heterogeneous conditions, in ionic liquids, and in supercritical CO2.
However, for certain applications and economic considerations, a low concentration of transition metal catalyst in an ATRP medium may be desired. Several methods have been developed to remove or reduce the amount of transition metals in the process, but these processes may add additional cost to the preparation of polymers by ATRP.
Several methods may be used to provide polymers by ATRP processes with low concentrations of catalysts. Such methods include performing an ATRP process with highly active catalyst that may require a lower concentration of catalyst to maintain the desired polymerization rate, for example, CuBr complexed by Me6TREN is ˜10,000 more active than CuBr complexed by bipyridine ligands; immobilizing the catalysts on solids such as a hybrid catalyst system comprising both immobilized catalyst complexes interacting with small concentrations of soluble catalysts (˜10-20 ppm); and several post polymerization methods developed to recover and regenerate catalysts, including separating the catalyst by filtration, adsorption, precipitation or extraction. For example, CuBr/PMDETA complex may be oxidized to Cu(II) species by exposure to air and quantitatively extracted from toluene to water, resulting, in some cases, with less than 1 ppm of catalyst remaining in the polymer. In spite of these advances, there remains a need to reduce the concentration of catalyst in the active polymerization media while maintaining polymer reaction rate and retaining control over MW and PDI.
The most attractive route may be just a simple decrease of the amount of the catalyst, providing that it has a sufficient reactivity. For example, ATRP processes comprising CuBr/Me6TREN complexes may be carried out at room temperature with much lower concentrations of the copper based catalyst. Regrettably, the amount of transition metal catalyst, such a Cu(I), may not simply be reduced 10,000 fold. Radical termination reactions result in an increase in the concentration of the transition metal catalyst in the deactivator state and irreversible consumption of the catalyst activators. In certain embodiments with certain monomers, the polymerization may stop if the amount of Cu(I) present in the reaction is below 10% of the initiator (as, 1˜10% of chains are terminated). The amount of terminated chains depends on the concentration of propagating radicals and rate constant of termination according to equation 1, which describes the number of terminated chains (or loss of Cu(I) activator) in an ATRP.−[CuI]=[Pt]=kt[P•]2t  (1)
The ATRP rate law (Equation 2) indicates that the polymerization rate depends on the ratio of Cu(I) to X—Cu(II) concentration but does NOT depend on the absolute concentration of copper complexes. Thus, in principle, the amount of copper may be reduced without affecting polymerization rate as long as the ratio of activator to deactivator is maintained.
                              R          p                =                                                            k                p                            ⁡                              [                M                ]                                      ⁡                          [                              P                *                            ]                                =                                                    k                p                            ⁡                              [                M                ]                                      ⁢                                                            K                  eq                                ⁡                                  [                  I                  ]                                            o                        ⁢                                          [                                  Cu                  I                                ]                                            [                                  X                  -                                      Cu                    II                                                  ]                                                                        (        2        )            
Unfortunately, as the reaction progresses the ratio of Cu(I) to X—Cu(II) is reduced through termination reactions and the polymerization rate decreases and eventually, in the absence of a sufficient concentration of the catalyst activator ATRP stops. See Equation 3. Thus, the amount of copper catalyst complexes that have generally been added to an ATRP reaction has exceed that of the expected number of terminated chains (i.e. >10% [I]o) in order to drive the reaction to completion.−Δ[CuI/L]=Δ[CuIIX/L]=Δ[Pdead]=kt∫[P•]2dt  (3)
Some amount of the deactivation species (i.e. X—Cu(II)) is also needed in the system for a well-controlled polymerization because molecular weight distribution and initial molecular weight depend on the ratio of propagation and deactivation rate constants and concentration of deactivator, according to Equation 4.
                                          M            w                                M            n                          =                  1          +                      1                          DP              n                                +                                    (                                                                                          [                                              R                        -                        X                                            ]                                        o                                    ⁢                                      k                    p                                                                                        k                    da                                    ⁡                                      [                                          X                      -                                              Cu                        II                                                              ]                                                              )                        ⁢                          (                                                2                  p                                -                1                            )                                                          (        4        )            
In a RAFT polymerization process termination reactions are suppressed through the addition of a suitable thiocarbonylthio compound, also known as a dithioester, to an otherwise conventional free radical polymerization; i.e. there is a continuous slow generation of radicals by decomposition of a standard radical initiator in order to drive the reaction forward. Control in such a RAFT process is thought to be achieved through a degenerative chain transfer mechanism in which a propagating radical reacts with the thiocarbonylthio compound to produce an intermediate radical species. This process decreases the instantaneous number of free radicals available for termination reactions that require two free radicals. RAFT (co)polymerization reactions have been discussed in U.S. Pat. Nos. 6,153,705; 6,380,355; 6,642,318 and 6,855,840.
There is a need for a transition metal catalyzed chain transfer polymerization process for free radically (co)polymerizable monomers that uses low concentrations of catalysts.